A. Field of the Invention
This invention relates to coherent light generators, and more particularly to plural active media or active media having plural dopants. The invention includes an optical fiber doped with thulium and holmium and europium, terbium, or a mixture thereof. The optical fiber is used in a four-level laser system having an optical gain about a wavelength of 1.47 .mu.m. A method for making and a method for using this optical fiber are also provided.
B. Description of the Related Art
A method of lasing involves exciting an active medium or optical fiber core by light energy. The light energy is produced by a source known as a "pump," which is typically coupled to the optical fiber to pump energy to ions in the core. In some situations, the pump energy may be absorbed by non-lasing energy levels. However, upon receipt of the energy some ions have electrons which are boosted to higher energy levels or "states." But an electron can only stay at higher energy levels for a limited amount of time before giving up its extra energy and falling to lower energy levels in what is termed a "transition." Also, a fluorescent (i.e., lasing) transition may be reabsorbed, resulting in "excited state absorption." Otherwise, however, this energy given up produces light at certain wavelengths--the laser light emitted by lasers.
Optical amplifiers, optical oscillators, superluminescence sources, medical lasers, and particularly optical communications systems have been designed to operate within certain wavelength ranges (i.e., "windows"), and one such window includes the 1.5 .mu.m wavelength for telecommunications applications. Erbium-based laser systems previously have been used to amplify such signals in telecommunications applications, but these have been operated as three-level laser systems. A three-level laser system has a transition that terminates in the ground state.
Three-level systems have several significant limitations, however. In a three-level system, the energy levels that fluoresce overlap those that absorb. Thus, a powerful light must be used to excite enough ions to operate the laser.
Four-level systems involve laser transitions which do not terminate in the ground state. Instead, the transitions terminate in a state that is initially unpopulated by electrons.
A four-level laser system has been made using fluoride glass doped with thulium to operate in a spectral region about 1.47 .mu.m. Fluorescence from such a system has a broad energy band extending from 1.40 .mu.m to 1.53 .mu.m and providing a widely tunable light source. This is useful for a broad band light amplifier, particularly for wavelength division multiplexed systems. Also, the 1.47 .mu.m wavelength produced by such a laser system is within another commonly used telecommunications window.
However, this approach has suffered from a problem: The electrons energized to a .sup.3 H.sub.4 energy leveling thulium have a longer lifetime then those occupying a .sup.3 F.sub.4 lower energy level. As a result, the electrons in the upper energy level descend to the lower level more quickly than the lower level is emptied. This problem is known as "terminal state bottlenecking" and results in self-terminating behavior of the laser light emission.
G. Rosenblatt, R. Ginther, R. Stoneman and L. Esterowitz, "Laser Emission at 1.47 .mu.m from Fluorozirconate Glass Doped with Tm.sup.3+ and Tb.sup.3+," a paper presented at the Tunable Solid State Laser Conference, Cape Cod, Mass (1989) (ROSENBLATT) suggested using terbium to eliminate this terminal state bottleneck. However, the suggested use of terbium not only resulted in a reduction of the terminal state population but also resulted in a reduction of the population of the upper laser level as well. Because laser light output is related to the ion population energized to the upper laser level, the operating efficiency of the ROSENBLATT laser was significantly diminished. Thus, this approach was not successful in producing an efficient, continuous-wave laser with the desired wavelength output.
Therefore, an efficient four-level laser system operable at the 1.47 .mu.m region has eluded those skilled in the art despite their concerted efforts, and solving the terminal state bottlenecking problem without significantly decreasing laser efficiency has posed a significant barrier in the development of such a four-level laser.